Many optical and mechanical systems require precise angular alignment. For example, equipment for electron beam and optical lithography includes moving stages in which the stage position is measured using interferometers that require precise alignment. Interferometric metrology is also used in precise machine tools. These and similar devices require mirrors carried on the moving stages to be aligned perpendicularly to a laser beam. Interferometers use other optical surfaces such as those of glass prisms which require accurate alignment. Using conventional methods, it is difficult to know accurately whether an interferometer axis is aligned, since its location in space depends on glass or mirror geometry and since it cannot readily be viewed in a manner that a laser beam is viewed. Misalignment of an interferometer can give rise to error terms of the first order in stage position and in first and second order in yaw and pitch, in accordance with conventional analysis.
A traditional method of aligning a reflective surface employs an autocollimator as an alignment tool. An autocollimator is configured basically as a telescope with a reticle, usually illuminated with a self contained light source. If an autocollimator is pointed at a distant flat mirror that is closely but not exactly perpendicular to the axis of the instrument, then an observer will observe through an eyepiece an image of the reticle that is slightly offset from the directly viewed reticle. When the reticle and its image superpose exactly, in focus, size and position, then light from the instrument is collimated and is perpendicular to the mirror. These devices are usually constructed to allow small angular displacements of the mirror to be measured, sometimes automatically. A disadvantage of an autocollimator is the requirement for an illuminated reticle, that must be precisely aligned with both the laser beam and the target reflector.
Another traditional instrument used for optical alignment is a sextant, which superposes the images of two objects, typically the sun and the horizon, for the purpose of measuring their apparent angular separation. Small motions of the instrument do not change the apparent relative positions of the objects. However, a sextant and similar devices depend on accurately movable mirrors and accurately calibrated circles, and generally cannot view sources located in opposite directions.
A traditional method recommended by metrological interferometer manufacturers to align mirrors and other reflective surfaces in a laser beam path is to place a white card at the laser output aperture with a hole in it just big enough to transmit the beam. The reflected beam from the mirror being aligned returns to the card, illuminating a visible spot on the surface of the card facing away from the laser. When the mirror is adjusted so that the spot appears centered on the hole, the mirror is perpendicular to the beam. This method suffers from several disadvantages: The laser beam may be several millimeters in diameter, making small errors hard to detect. The offset of the spot is twice the angular error of the mirror multiplied by the distance between the card and the mirror, which may be quite short, often less than a meter, and there may be little geometric magnification and no optical magnification of the error. The two spots being superposed may be large and are on opposite sides of the card. The hole in the card prevents the centers of the spots from being seen. The card is often not conveniently visible from the location where the adjustment is being made.
It is desirable in the art to provide a method and apparatus to determine accurately and simply whether optically reflective surfaces are in accurate alignment with a laser beam or similar parallel light beam. A null method is most desirable, which gives an unambiguous indication of accurate alignment independent of calibration, and in which any error of alignment is magnified. It is desirable that the accuracy not be degraded when the laser beam diameter is large. It is desirable that the apparatus not disturb the laser or system optics to be aligned and that removal of the apparatus from the system being aligned not disturb its alignment. These requirements do not preclude the apparatus being a permanent part of the system, nor do they preclude the apparatus providing a calibrated measurement of error, should any exist.